Rework PHP API intro

[adriendupuis]

Question	Answer
JIRA Ticket	IBX-4929
Versions	4.5, 4.6, master
Edition	All

Rework "Using API services"

• reorder 1) intro to PHP API using services, 2) Service list, 3) How to access those services.
• list all possible ways to call a service
• add links to class files on GitHub (will be replaced by PHP API Ref in the future)

Preview: https://ez-systems-developer-documentation--2386.com.readthedocs.build/en/2386/api/php_api/php_api/#using-api-services

Note: This was originally done in #1904 but must be discussed before.

Checklist

• Text renders correctly
• Text has been checked with vale
• Description metadata is up to date
• Redirects cover removed/moved pages
• Code samples are working
• PHP code samples have been fixed with PHP CS fixer
• Added link to this PR in relevant JIRA ticket or code PR

[lserwatka]

@adriendupuis @mnocon Is it possible to change the domain name from ez-systems to ibexa? Shall we rebrand Read The Docs account?

[adriendupuis]

@adriendupuis @mnocon Is it possible to change the domain name from ez-systems to ibexa? Shall we rebrand Read The Docs account?

@lserwatka

We can't change existing projects' URLs (https://docs.readthedocs.io/en/stable/faq.html#how-do-i-change-my-project-slug-the-url-your-docs-are-served-at).
We can't create new project with already used repository. So we got to delete the current projects to recreate them.
We have to study what such deletion will break. (like opened PRs' previews?)